This invention relates to washing machines, and more particularly to speed control circuits for regulating the rotating speed of the rotatable drum of a washing machine.
In washing machines such as those commonly found in the home, the quality of the wash depends upon the drum speed of the washing machine remaining relatively constant (i.e., within three percent or so of its nominal value) even though conditions may vary widely. For example, the amount of clothes and the amount of water within a washing machine drum vary from wash to wash and even during a wash cycle. If the drum speed is not held relatively constant despite these varying conditions, the quality of the wash degrades as the drum speed changes with different amounts of water and clothes present or when their distribution in the drum changes during the wash cycle. It is therefore a problem to maintain the desired drum rotating speed throughout the wash cycle or from wash to wash.
Methods of regulating drum speed currently being used involve either direct speed sensing, sensing of motor currents or open loop compensation. Use of sensors is undesirable because they greatly add to the cost of the control circuitry within the washing machine and/or lower the reliability of the system. Open loop compensation by simply moving up and down the voltage-frequency curve is not accurate enough, particularly when variations in the washing machine operating parameters are taken into account.